This invention relates to an encoder for encoding a signal element sequence into a code sequence to which a compression encoding scheme is selectively applied and to a decoder for decoding the code sequence into a reproduction of the signal element sequence to which an expansion decoding scheme which corresponds to the compression encoding scheme is selectively applied.
An original sequence of signal elements, such as pels or picture elements, are encoded into a sequence of codes either for transmittal through a transmission medium or recordation on a recording medium. Generally signal compression is applied to reduce the amount of information in the code sequence. The original sequence is encoded into the code sequence usually in two steps. In a first step, the original sequence is encoded into a redundancy reduced sequence of various signal elements. In a second step, the redundancy reduced sequence is encoded into the code sequence.
Various techniques are already known for use in carrying out the first step. By way of example, predictive encoding is described in an article contributed by J. C. Candy et al. to BSTJ, Vol. 50, No. 6 (July-August 1971), pages 1889 to 1917, under the title of "Transmitting Television as Clusters of Frame-to-Frame Differences." Orthogonal transformation is described in another article contributed by Wen-Hsiung Chen et al to IEEE Transactions on Communications, Vol. COM-25, No. 11 (November 1977), pages 1285 to 1292, and entitled "Adaptive Coding of Monochrome and Color Images." The second step is carried out typically by Huffman encoding.
In the redundancy reduced sequence, the signal elements comprise clusters of significant or information bearing signal elements and runs of insignificant or background signal elements. The clusters and runs are arranged to follow one another in an alternating fashion. The significant signal elements have various finite or significant amplitudes or levels and may include or be constructed of predictive errors and orthogonal transform coefficients used in the predictive encoding and in the orthogonal transformation. The insignificant signal elements, as herein called, represent background data for the significant signal elements and have a substantially constant amplitude or a zero level amplitude or a black level. Briefly speaking, the redundancy reduced sequence has a statistical characteristic of concentating at the insignificant signal elements.
As known in the art, the runs have run lengths, respectively. On producing the code sequence, each run is encoded into a run length code representative of the run length of the run under consideration. Each cluster is encoded into a succession of significant codes representative of the respective significant signal elements of the cluster in question.
The redundancy reduced sequence is encoded into such a code sequence with selective application of a compression encoding scheme to the significant signal elements of the clusters. In relation to the compression encoding scheme, the significant signal elements of some of the clusters are subsampled into subsamples leave remaining signal elements. The remaining signal elements or subsamples are encoded into a reduced number of significant codes in the code sequence.
Inasmuch as the compression encoding scheme is selectively applied to the clusters, application and non-application of the compression encoding scheme must be indicated in the code sequence by a certain code which may be called a mode code. The signal element sequence is generally divisible into a plurality of subsequences, such as scanning lines used in television or facsimile. If the mode code were used only once for each subsequence as in "subsample mode" and "subline mode" described in an article contributed by Hiroshi Yasuda et al to IEEE Transactions on Communications, Vol. COM-25, No. 5 (May 1977), pages 508 to 516, under the title of "Transmitting 4-MHz TV Signals by Combinational Difference Coding," the code sequence could be subjected to only a negligible increase in the amount of information. The mode codes would, however, have to be used for each cluster so as to indicate whether or not the cluster under consideration is encoded with application of the compression encoding scheme. This gives rise to a considerable increase in the amount of information in the code sequence. Alternatively, application and non-application of the compression encoding scheme must separately be encoded. This requires an additional encoder and results in an increase in hardware.